1. Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine, more particularly to an air/fuel ratio control system for a direct injection spark ignition engine, in which gasoline fuel is directly injected into the combustion chamber of the engine.
2. Description of the Related Art
In the internal combustion engine, since the air-fuel mixture varies according to the cylinder temperature and the air/fuel ratio is a decisive factor to the cylinder temperature, it is known, from Japanese Laid-Open Patent Application No. Hei 5 (1993)-79374, for example, to correct the desired air/fuel ratio using a charging efficiency correction coefficient (for adjusting the charging efficiency of intake air) retrieved from table data (prepared beforehand) using the desired air/fuel ratio itself, and then to correct the basic fuel injection amount by the corrected desired air/fuel ratio to determine the output fuel injection amount. In this prior art, two kinds of table data are predetermined such that one of them is selected based on the engine speed.
Aside from the above, a direct injection spark ignition engine has recently been proposed in which gasoline fuel is directly injected into the combustion chamber such that an ultra-lean burn combustion or a stratified combustion (in an ultra lean air/fuel ratio) or the pre-mixture charged combustion (in a uniform air/fuel ratio) occurs in the engine as is disclosed in, for example, Japanese Patent Publication No. Hei 4 (1992)-37264.
In such a direct injection spark ignition engine, since the cylinder temperature is different with the form of combustion, the aforesaid prior art (Japanese Laid-Open Patent Application No. Hei 5 (1993)-79374) is not effective in determining the charging efficiency correction coefficient to be used for correcting the desired air/fuel ratio adequately, thereby making it difficult to determine the fuel injection amount appropriately.
An object of the present invention is therefore to provide an air/fuel ratio control system for an internal combustion engine, more particularly for a direct injection spark ignition engine which can determine the charging efficiency correction amount to be used for correcting the desired air/fuel ratio adequately and hence, can determine the fuel injection amount appropriately.
Further, the cylinder temperature varies according to the presence or absence of the operation of EGR (Exhaust Gas Recirculation) in which the exhaust gas is partially recirculated in the intake system of the engine.
An object of the present invention is therefore to provide an air/fuel ratio control system for an internal combustion engine, more particularly for a direct injection spark ignition engine which can determine the charging correction coefficient to be used for correcting the desired air/fuel ratio adequately, irrespective of the presence or absence of the EGR operation and hence, can determine the fuel injection amount appropriately.
This invention achieves this object by providing a system for controlling an air/fuel ratio for a direct injection spark ignition engine which is operated at one of two combustion forms including an ultra-lean burn combustion and a pre-mixture charged combustion; comprising: engine operating condition detecting means for detecting operating conditions of the engine at least including an engine speed and an engine load; basic fuel injection amount determining means for determining a basic fuel injection amount based at least on the detected engine speed and the engine load of the engine operating conditions; desired air/fuel ratio determining means for determining a desired air/fuel ratio of exhaust gas produced by the engine; combustion form discriminating means for discriminating at which form of combustion the engine is operated; charging efficiency correction coefficient determining means for determining a charging efficiency correction coefficient for adjusting a charging efficiency of intake air based at least on the determined desired air/fuel ratio and the form of combustion at which the engine is operated; desired air/fuel ratio correcting means for correcting the desired air/fuel ratio based on the determined charging efficiency correction coefficient; output fuel injection amount determining means for determining an output fuel injection amount by correcting the basic fuel injection amount at least by the corrected desired air/fuel ratio; and fuel injecting means for injecting fuel into a cylinder of the engine based on the determined output fuel injection amount.